Manufacture of resistance units



March 1, 1932. L JONES ET AL 1,847,653

MANUFACTURE OF RESISTANCE UNITS Filed March 12, 1928 INVENTOR sTer L. Jones Joseph A.F|onzer I BY / ATl' N YS Patented Mar. 1, 1932 UNITED STATES PATENT OFFICE LESTER L. JONES, OF ORADELL, NEW JERSEY, AND JOSEPH FLANZER, OI BROOKLYN, NEW YORK, ASSIGNOBS TO TECHNIDYNE CORPORATION, OF NEW YORK, N. Y, A.

CORPORATION OF NEW YORK MANUFACTURE OF RESISTANCE UNITS Application filed March 12, 1928. Serial No. 280,987; I

This invention relates to the art of making resistances, and relates more particularly to the manufacture of resistance units having painted or coated resistance films or strips; and has special reference to the making of resistance elements of this nature having predetermined or desired temperature coefiicients.

In the copending application of Lester L. Jones et al., Ser. No. 61,326, filed Oct. 8, 1925, and which issued as Patent 1,762,990, June 10, 1930, there is disclosed the art of making a resistance paint capable of being applied to 'SllCh insulating substances as glass, porcelain, bakelite and the like and capable of being applied by a coating or painting process to produce resistance units or elements, the coating of resistance material comprising finely divided graphite held in a binder such as an alkaline silicate. These resistance coatings when thus applied form very thin resistance films having a thickness of the order of magnitude of 0.25 thousandths of an inch, which resistance films are produced in continuous and unbroken coatings having a comparatively low specific resistance. In another copending application of Lester L. Jones et al., Ser. No. 167,583, filed Feb. 11, 1927, which issued as Patent 1,773,105, Aug. 19, 1930, there is disclosed an improved resistance paint and method of'making the same also embodying a finely divided graphite held in a binder which may be coated or painted on mica, glass and other bases to form thin non-hygroscopic resistance films.

These graphite resistance paints or coatings possess a negative temperature coefficient which becomes pronounced at relatively high operating temperatures; and this may become objectionable, particularly where it is desired to employ the resistance units in circuits where it is necessary to. maintain the resistance values constant with a substantial degree of accuracy, as within 1%, and over a large range of operating temperatures, as for example, between room temperature and 200C. The principal object of our present invention is directed to the making of resistance units or elements, employing these graphite paints or coatings, in which the negative temperature coefiicient may be compensated for or eliminated so that the resistance values of the units may be maintained substantially constant over wide temperature variations, or more broadly in which'the temperature coefiicient may be fixed at any desired low value, either positive or negative, toproduce desired variations of the resistance value with variations in temperature over a suitable range.

To the accom lishment of the foregoing and such other 0 jects as will hereinafter appear, our invention consists in the elements and their relation one to the other as more particularly described hereinafter and sought to be defined in the claims; reference beln had to the accompanying drawings, whic show the preferred embodiment of our invention, and in which:

Figs. 1 and 2 are views showing steps ofv making resistance units embodying the principles of our invention, Fig. 2 showing the reslstance unit in completed form.

We have found that the resistance elements or units employing painted or coated graph- 1te films may be made with predetermined low negative or positive temperature coefficients or may be made with a substantially,

zero temperature coeflicient by suitably combimng with a film or coating of the graphite paint (possessing the negative temperature coefficient) a second film or coating of a ma terial having a positive temperature coefliclent, the two coatings being so related that the temperature coefficient of one film may be compensated or neutralized in whole or 1n part by the opposite temperature coeflicient of the other film.

This we accomplish by constructing a reslstance unit in two resistance sections having opposite temperature coeflicients and by then selecting .or predetermining the ratio of the resistance values of the two resistance sections in such a manner as to produce a predetermined temperature coeflicient and a resulting resistance value of a desired magni-' tude for the composite unit. In the practice of our invention, referring now inore in detail to the drawings, we first coat or aint a suitable insulating base such as 10 wit a film or layer 11 of the graphite resistance paint and we then deposit, preferably by electroplating, a second film or layer 12 of a metal such as nickel over only a portion or part of the resistance paint film 11, these layers thus forming two separate resistance sections 13 and 14. The total resistance value of the composite unit as well as the ratio of the resistance values of the sections 13 and 14 are then predetermined by arranging the resistance sections into given or selected lengths, as will be described further hereinafter.

Preferably we employ for the first film or layer 11 the paint or composition described and claimed in the foresaid patent to Lester L. Jones et al. 1,773,105, this resistance paint being composed of a finely divided graphite mixed with a binder and vehicle consisting of a metallic phosphate. In compounding the resistance material, the graphite, preferably a colloidal graphite, is mixed with a solution of aluminum in phosphoric acid to form a freely flowing paint. The aluminum in the phosphoric acid produces an aluminum phosphate; and in the preparation of the aluminum phosphate it has been found desirable to add aluminum to the phosphoric acid in such an amount as ust fails toproduce a precipitate. Addition of more than this amount, which would produce a precipitate, is undesirable because of the presence in the paint of large particles formed by the precipitate which interefere with the production of a smooth uniform film. On the other hand, the addition of an insuflicient quantity of aluminum or aluminum phosphate is'undesirable mainly because of the more highly acid condition of the resulting paint, which tends to destroy the colloidal nature of the graphite, causing clotting of the paint with streakiness in the film. It is therefore preferred to use a solution of aluminum in phosphoric acid in a slight excess of phosphoric acid, aluminum being added in such an amount as just fails to produce a precipitate. The aluminum phosphate may also be prepared by the addition of C. P. aluminum phosphate to phosphoric acid in sufficient quantity to just avoid the formation of any precipitate.

The paint thus compounded is then applied to the insulating base 10, which may be. a glass or porcelain rod or a glazed or enameled surface, or may be applied to mica to form resistance coatings of very thin films of the order of magnitude of 0.1 to 0.25 thousandths of an inch, such coatings being formed in continuous and unbroken layers, as shown for example in Fig. 1 of the drawings. The paint may be applied to the insulating ma terial by means of a brush or by dipping or by any other method. After the bases, such as 10, are coated with paint and are air-dried,

the painted unit is preferably heated to approximately 500 C. for a period of approximately one hour. The heat treatment causes a reduction of the electrical resistance in the film to approximately 1% of its air-dried value. The coating or film thus formed is tough and durable and may be made more or less glassy, depending upon the relative amounts of aluminum phosphate and colloidal graphite used. In lieu of colloidal graphite other forms of finely divided graphite may be employed with good results, such as finely divided graphite commercially known as air floated graphite.

lVe have found that these graphite resistance paints or coatings may be plated with a metallic coating such as 12, and that the plating may be carried out without affecting the integrity or condition of the painted coating 11. More particularly, we have discovered that with a nickel plating solution the electroplatin step is not critical as to the plating period nor as to the voltage or temperature of the plating bath, and that the binder of the resistance paint or film 11 is insoluble in the plating solution, these being highly important advantages of the process. The plated coating 12 produced with a nickel plating solution is therefore applied over the paint coating 11 to produce the resistance section 14, the ratio of the areas of the resistance sections 13 and 14 being determined in a manner hereinafter to be described. In the electroplating step the anodes are preferably symmetrically disposed with respect to the surface to be plated, this so as to obtain a homogeneous deposition of the nickel. The thickness of the electrodeposit of the nickel film is preferably of the same order of magnitude as that of the paint film, and is about 0.2 thousandths of an inch. The plating solution is substantially neutral to the paint film 11 and hence the paint film is unattacked, and we have found it unnecessary to treat the units to remove traces of any chemicals of the plating bath. Moreover, the paint film 11 which adheres very firmly to the insulating base 10 forms a firm basis or support for the deposited or plated metal film. Fig. 1 of the drawings shows the embryo resistance element produced by the steps thus far described, the thicknesses of the films being exaggerated for purposes of elucidation.

To predetermine the temperature coefiicient for the resistance element, the resistance values of the sections 13 and 14 of the unit are predetermined so as to have a given ratio and a desired additive or resulting resistance. To permit of a uniform heat dissipation over both sections of the unit, the respective areas of the sections are also given a selected ratio. \Ve have found that the best results are achieved by making the ratio of the resistance values of the two sections equal to the inverse ratio of the temperature coefiicients of the films taken over the range of temperatures in which the units are to be used. Thus with a range of temperature variation between room temperature and 160 C. the negative temperature coefficient of the paint film 11 is about 15%, while the positive temperature coefiicient of the plated film 12 is about 50%. The resistance values of the sections 14 and 13 are therefore selected to have a ratio of 15 to 50. The areas of the sections 13 and 14 are preferably determined by the resistance values of the sections at the average operating temperature thereof. Thus the areas are given the same ratio as the resistance values of the sections at say an operating temperature of 160 C. In the example stated, the areas of the sections 14 and 13 will have a ratio of about 7 to 13. The unit thus constructed is designed for complete neutralization of the negative temperature coefficient by the positive temperature coefficient; and it is found that the unit is noncritical in that the same may be operated within a range of 140 to 180 C. with only small deviations from zero temperature co efficient, the deviations over the entire range from room temperature to 160 (J. being of the order of 1%.

The preferred way of imparting the desired resistance values to the resistance sections 14 and 13 to obtain the desired resistance ratio and resultant is by employing the spiralling method disclosed and claimed in the patent to Lester L. Jones, No. 1,635,184 of July 12, 1927. In accordance with this method, the insulating base is made in cylindrical form and the resistance layer thereon is provided witha helical groove cut along the base, the helix being cut to a length for producing a resistance of a given or desired value. Applying this method, both the sections 14 and 13 are cut helically, preferably in a lathe, so as to provide the helical strip 16 for the section 14 and the helical strip 17 for the resistance section 13, as shown in Fig. 2 of the drawings. Due to the fact that the specific conductance of the resistance layer 12 is much greater than that of the paint film 11, the pitch of the helix strip 16 is much smaller than that of the helix strip 17, all as depicted in Fig. 2 of the drawings. It will be understood that in cutting the helical grooves the resistance values of the two sections of the element are first determined and then obtained after the manner disclosed in said Patent No. 1,635,184.

If the resistance units are made on enameled iron or enameled brass or copper, the ratio of areas need not be selected as hereinbefore set forth, since the heat conduction through the metal base tends to equalize the temperatures of the nickel and paint sections. The aforesaid desired ratio of resistance areas is therefore preferred only when the heat dissipation is by radiation or convection. Slight variations in the area ratio serve to change the tem rature at which the total resistance is exactl the same as the initial resistance (at room temperature). The completed resistance element is provided with contact terminals in the form of metallic bands 18 and 19 which are clamped over the ends of the paint resistance film 11, as shown in Fig. 2 of the drawings.

The arithmetic example heretofore given for the resistance ratios and ratio of resistance areas is for a resistance unit having a zero temperature coefficient. It will be understood that a variation from this resistance ratio on one side or the other will produce either a predetermined negative or a predetermined positive temperature coeflicient for the unit. The principles of the invention may therefore be employed in producing resistance elements havin small and intended values of resistance variations with temperature either in a positive direction or in a negative direction.

The manner of making and using resistance units embodying our invention and the many advantages thereof will in the main be fully apparent from the above detailed description thereof. It will be further appar ent that while we have shown and described our invention in the preferred form, many changes and modifications may be made in the structure disclosed without de a-rting from the spirit of the invention, defined in the following claims.

We claim:

1. The method of manufacturing a nonhygroscopic high resistance unit having a desired temperature coefficient, which includes coating the surface of an insulation base with a high resistance coating comprising colloidal graphite suspended in a vehicle including a metallic phosphate in an excess of phosphoric acid, baking the resulting unit. to form a high resistance film having a negative temperature coefficient, and coating a portion of the graphite surface with a thin film of metallic nickel in order to form a resistance film having a positive temperature coefiicient, the graphite and nickel resistance films being relatively proportioned to obtain the desired over-all temperature coefficient, and being further proportioned to obtain an over-all series resistance equal to the desired resistance of the unit.

2. The method of manufacturing a nonhygroscopic high resistance unit having a zero temperature coefiicient, which includes coating the surface of an insulation base with a high resistance coating comprising finely divided graphite suspended in a vehicle including a metallic phosphate, baking the resulting unit to 'form a high resistance film having a negative temperature coefficient, and coating a portion of the graphite surface with a thin film of metal in order to form a resistance film having a positive temperature coefficient, the graphite and metal resistance films being proportioned to obtain graphite and metal resistance values inversely proportional to the respective temperature coefficients at a predetermined operating temperature, and an overall series resistance equal to the desired resistance of the unit.

The method of manufacturing a nonhygroscopic high resistance unit having a zero temperature coefficient, which includes coating the surface of an insulation base with a high resistance coating comprising finely divided graphite suspended in a vehicle including a metallic phosphate and baking the resulting unit to form a high resistance film having a negative temperature coefficient, coating a portion of the gra hite surface with a thin film of metal in or er to form a resistance film having a positive temperature coefficient, and thereafter appropriately cutting the graphite and metal resistance films to obtain graphite and metal resistance values inversely proportional to the respective tempertaure coefficients at a predetermined operating temperature, and an over-all series resistance equal to the desired resistance of the unit.

4. The method of manufacturing a nonhygroscopic high resistance unit having a zero temperature coefficient, which includes coating the entire external surface of a cylindrical insulation base with a high resistance coating comprising colloidal graphite suspended in a vehicle including a metallic phosphate in an excess of phosphoric acid and baking the resulting unit to form a high resistance film having a negative temperature coefficient, electro-plating a portion of the coated surface with a thin film of metallic nickel in order to form a resistance film having a positive temperature coefficient, and thereafter spiral cutting the graphite and nickel resistance films at pitches appropriate to obtain graphite and nickel resistance values inversely proportional to the respective temperature coefficients at a predetermined operating temperature. and an over-all series resistance equal to the desired resistance of the unit.

5. The method of manufacturing a. nonhygroscopic high resistance unit having a zero temperature coefficient, which includes coating the surface of an insulation base with a high resistance coating comprising colloidal graphite suspended in a vehicle including a metallic phosphate in an excess of phosphoric acid and baking the resulting unit to form a high resistance film having a negative temperature coefficient, and plating a portion In of the coated surface with a thin film of metallic nickel in order to form a resistance film having a positive temperature coefficient. the areas of the plated and unplated portions of the resistance unit being proportional to the desired respective resistance values, the

resistance values being inversely pro ortional to the respective temperature coe cients at a predetermined operating temperature, and the over-all series resistance being equal to the desired resistance of the unit.

6. A non-hygroscopic high resistance unit having a, desired temperature coefficient comprising an insulation base the surface of which is coated with a thin film of colloidal graphite held in a binding metallic phosphate vehicle and forming a high resistance film having a negative temperature coefficient, and a thin film of metallic nickel plated over only a portion of the resultin graphite film in order to form a resistance fi m having a positive temperature coefficient, the resulting resistance films being properly relatively proportioned to obtain the desired over-all temperature coefficient, and being further proportioned to obtain an over-all series resistance equal to the desired resistance for the unit.

7. A non-hygroscopic high resistance unit having a zero temperature coefficient comprising an insulation base the surface of which is coated with a thin film of finely divided graphite held in a binding metallic phosphate vehicle and forming a high resis tance film having a negative temperature coefficient, and a thin film of metal coated over only a portion of the resulting graphite film in order to form a resistance film having a positive temperature coefficient, the resulting resistance films being properly proportioned to obtain resistance values inversely proportional to the temperature coefficients at a predetermined operating temperature, and an over-all series resistance equal to the desired resistance for the unit.

8. A non-hygroscopic high resistance unit having a zero temperature coefficient comprising an insulation base the external surface of which is coated with a thin film of finely divided graphite held in a binding metallic phosphate vehicle and forming a high resistance film having a negative temperature coefficient, and a thin film of metal coated over only a portion of the resulting graphite film in order to form a resistance film having a positive temperature coefficient, the resulting resistance films being cut to obtain resistance values inversely proportional to the temperature coefficients at a predetermined operating temperature, and an over-all series resistance equal to the desired resistance for the unit.

9. A non-hygroscopic high resistance unit having a zero temperature coefficient comprising a cylindrical insulation base the external surface of which is coated with a thin film of colloidal graphite held in a binding metallic phosphate vehicle and forming a high resistance film having a negative temperature coefficient, and a thin film of metallic nickel plated over only a portion of the resulting graphite film in order to form a resistance film having a positive temperature cocflicient, the resulting resistance films being spirally cut at pitches properly selected to obtain resistance values inversely proportional to the temperature coefficients at a predetermined operating temperature, and an over-all series resistance equal to the desired resistance for the unit.

10. A non-hygroscopic high resistance 113?; having a zero temperature coefiicient co prising an insulation base the surface of which is coated with a thin film of colloidal graphite held in a binding metallic phosphate vehicle and forming a high resistance film having a negative temperature coefiicient, a thin film of metallic nickel plated over only a portion of the resulting graphite film in order to form a resistance film having a positive temperature coefiicient, the areas of the plated and unplated portions of the resistance unit being proportional to the respective desired resistance values, the resistance'values being inversely proportional to the temperature coefiicients at a predetermined operating temperature. and the over-all series resistance being equal to the desired resistance for the unit.

Signed at New York in the county of New York and State of New York this 5th day of March A; D. 1928.

LESTER L. JONES. JOSEPH A. FLANZER. 

